This invention relates to an antenna mainly mounted on a portable radio apparatus and in particular to an antenna apparatus capable of providing a good radiation characteristic even in a state in which a portable radio is brought close to a human body.
In recent years, a compact, lightweight, and slim radio has been required as a radio apparatus for business use mainly operated in a VHF (very high frequency) band, and a small-sized portable radio apparatus as compared with the wavelength has been used. A normal mode helical antenna (NHA, hereinafter) of a quarter wavelength monopole antenna shape is generally available as a portable radio antenna for business use in a related art, and a small-sized antenna system excellent in portability kept out of the way of the user at the carrying time is provided.
FIG. 10 shows a portable radio apparatus. An NHA 20 wound in helical is electrically connected at one end to a radio apparatus main unit 21, where by a compact and lightweight antenna system is provided. Generally, the NHA 20 is designed so that the electric length becomes a quarter wavelength to match a radio circuit, and is matched to any desired resonance frequency in a free space. If the electric length of the NHA 20 does not become a quarter wavelength, the NHA 20 is designed so as to provide sufficient radiation performance in the free space by inserting a matching circuit between the NHA 20 and a radio circuit for matching.
However, if the NHA 20 shown in FIG. 10 is brought close to a human body for use, it receives the effect of the human body medium having a high dielectric constant and the radiation performance is largely degraded; this is a problem. Unlike a portable telephone, as shown in FIG. 11, a portable radio for business use is always fixed to a belt of the user so that it does not stand in the way of the radio user who works. In this state, the user communicates through a microphone and an earphone connected to the radio apparatus main unit. In this case, the NHA mounted on the radio apparatus main unit is brought extremely close to the belly of the human body and thus the antenna characteristic is strongly affected by the human body.
Hitherto, the effect of the positional relationship between the human body and the antenna on the antenna characteristic has been examined in various manners, and it is made clear that as the antenna is brought close to the human body, the gain is lowered. As the antenna is brought close to the human body, the input impedance of the antenna changes and does not match that of a circuit and a loss is caused by reflection. Further, a part of electromagnetic wave radiated from the antenna is absorbed in the antenna itself or the human body, causing a loss.
Conventionally, the radiation characteristic of the antenna has been based on the result considering only some electromagnetic waves radiated from the antenna and thus the loss caused by the impedance mismatch and the loss caused by absorbing the electromagnetic waves in the antenna or the human body has not been considered. However, the percentage occupied by the losses may be large depending on the frequency band and the antenna type.
As for an NHA reduced to 0.1 wavelength or less put to use in a 150-MHz band particularly, for example, like an antenna mounted on a portable radio for business use, the input impedance of the NHA in the proximity of the antenna changes sharply and thus a correct evaluation needs to be performed on the loss caused by the impedance mismatch. Further, such an NHA, which is small-sized, has small radiation resistance and therefore it is also necessary to consider the losses of high-frequency resistance of the metal line forming the antenna and a matching circuit.
As described above, to examine the radiation characteristic of the antenna, the percentage occupied by lost power in radiated electromagnetic waves varies depending on the antenna type as well as the frequency band and the lost power composition ratio also varies. Therefore, to improve the radiation characteristic of the antenna and design an antenna apparatus having a high gain, the factor of occurrence of lost power needs to be considered to minimize the lost power.
It is therefore an object of the invention to provide a high-gain antenna apparatus with a power loss minimized considering the occurrence factor of lost power.
According to a first aspect of the invention, an antenna apparatus in a portable radio apparatus comprises an antenna element mounted which receives electric power from a radio circuit in the portable radio apparatus and matching in a predetermined frequency band, and a matching circuit which matches impedance of the antenna element and internal impedance of the radio circuit, wherein the matching circuit has a match characteristic to conjugately match the internal impedance of the radio circuit and input impedance of the matching circuit.
According to a second aspect of the invention, the antenna apparatus according to the first aspect of the invention further comprises a match characteristic controller for varying the match characteristic of the matching circuit in response to change in the impedance of the antenna element.
According to a third aspect of the invention, an antenna apparatus according to the second aspect of the invention, wherein the match characteristic controller varies load of the matching circuit in response to change in the impedance of the antenna element to selectively adapt to at least the impedance of the antenna element when the antenna element is brought close to a human body with a sufficiently small spacing relative to the wavelength of the predetermined frequency band and the impedance of the antenna element when the antenna element is in a free space.
According to a fourth aspect of the invention, an antenna apparatus according to the second aspect of the invention, wherein the match characteristic controller varies capacitance load of the matching circuit in response to change in the impedance of the antenna element.
According to a fifth aspect of the invention, an antenna apparatus according to the first aspect of the invention, wherein the matching circuit includes a first matching circuit having a match characteristic to conjugately match the internal impedance of the radio circuit and the input impedance of the matching circuit when the portable radio is brought close to a human body with a sufficiently small spacing relative to the wavelength of the predetermined frequency band.
According to a sixth aspect of an antenna apparatus according to the fifth aspect of the invention, wherein the matching circuit further includes a second matching circuit having a match characteristic to conjugately match the internal impedance of the radio circuit and the input impedance of the matching circuit in a free space, and a selector which selects the first matching circuit or the second matching circuit in response to change in the impedance of the antenna element.
According to a seventh aspect of the invention, an antenna apparatus according to second aspect of the invention further comprises a mismatch detector which monitors electric power supplied to the antenna element and reflected because of a mismatch and detecting change in the impedance of the antenna element, wherein the match characteristic controller is controlled in response to the detection result of the mismatch detector.
According to an eighth aspect of the invention, an antenna apparatus according to the sixth aspect of the invention further comprises a mismatch detector which monitors electric power supplied to the antenna element and reflected because of a mismatch and detecting change in the impedance of the antenna element, wherein the selector is controlled in selection in response to the detection result of the mismatch detector.
According to the first aspect of the invention, the internal impedance of the radio circuit and the input impedance of the matching circuit are conjugately matched to each other, whereby the mismatch loss caused by the impedance mismatch of the antenna element can be decreased.
According to the second aspect of the invention, the match characteristic control means can vary the match characteristic of the matching circuit in response to change in the impedance of the antenna element, so that the match characteristic can be changed in response to the use state of the portable radio installing the antenna apparatus.
According to the third aspect of the invention, the match characteristic control means can switch the load of the matching circuit, whereby a plurality of match characteristics including the match characteristic corresponding to the impedance of the antenna element when the antenna element is brought close to a human body and the match characteristic corresponding to the impedance of the antenna element when the antenna element is in a free space can be selectively adapted in response to change in the impedance of the antenna element.
According to the fourth aspect of the invention, the capacitance load of the matching circuit is varied continuously in response to change in the impedance of the antenna element, whereby the match characteristic can be continuously adapted in response to change in the impedance of the antenna element.
According to the fifth aspect of the invention, the match characteristic of the matching circuit is set so that the internal impedance of the radio circuit and the input impedance of the matching circuit conjugately match each other in a state in which the antenna element is brought close to a human body, whereby the mismatch loss caused by the impedance mismatch of the antenna element can be decreased in a state in which the portable radio is carried for use.
According to the sixth aspect of the invention, the first matching circuit set so that the internal impedance of the radio circuit and the input impedance of the matching circuit conjugately match each other when the antenna is brought close to a human body and the second matching circuit set so that the internal impedance of the radio circuit and the input impedance of the matching circuit conjugately match each other in a free space are selectively switched in response to change in the impedance of the antenna element, whereby the mismatch loss caused by the impedance mismatch of the antenna element can be decreased both when the antenna is brought close to a human body and when the antenna is in a free space.
According to the seventh aspect of the invention, an impedance mismatch of the antenna element is detected and the match characteristic control means can be controlled based on the detection result for varying the match characteristic.
According to the eighth aspect of the invention, an impedance mismatch of the antenna element is detected and the selection means is controlled in selection based on the detection result for varying the match characteristic.